Billi, I also have olfactory hallucinations. For me, there are some that do preclude a migraine headach. But I also have experienced them when having flashbacks. I also experience auditory hallucinations such as hearing a TV on in the background when there was nothing turned on.

For you, if the smell is there only before sleep, I would suspect it may be a part of a flashback. You can try to do a little desensitization on your own by closing your eyes when you smell it and move them slowly left to right as if scanning a book. I started to do this with my PTSD related sensations and it GREATLY reduces the emotional charge!